A simple model for cellulose solubility in supercritical water

A simple model for the hydration of a polar molecule is developed in order to provide a description of cellulose solubility in water under ambient and supercritical conditions. The change in free energy upon hydration is regarded as the sum of the energy cost of forming a cavity and a polar contribution. The model is able to predict the existence of an optimal density for dissolution of polar solutes in supercritical water. Those results are in line with earlier experiments and simulations showing that water at high temperature and pressure dissolves cellulose, and that an optimal density for dissolution exists. The present study shows that the density dependence comes from the fact that both the cavity formation energy and the polar energy are highly density dependent but with opposing behaviour. The cavity formation energy increases with density, whereas the polar energy decreases. Based on the present model, it is possible to rationalize a few basic strategies regarding cellulose dissolution in aqueous media. To increase solubility, one can either increase the polar/electrostatic contribution, or more importantly, one can decrease the cost of cavity formation, e.g. by introducing co-solvents, changing temperature and/or pressure.